Wednesday, April 30, 2014

A paper published today in the Journal of Geophysical Research Atmospheres finds a link between solar cycles, the natural quasi-biennial oscillation [QBO], and the late winter polar vortex. The authors use ozone as a marker of "modulations of the winter Arctic stratosphere by the quasi-biennial oscillation (QBO) and the solar cycle. It is found that both the QBO and solar forcing in low latitudes can perturb the late winter polar vortex, likely via planetary wave divergence, causing an early breakdown of the vortex in the form of Stratospheric Sudden Warming [SSW]." The QBO and Sudden Stratospheric Warmings have in turn been linked to solar activity, suggesting that the Sun could be the ultimate source of polar vortex/jet stream blocking variability.

The total column ozone (TCO) observed from satellites and assimilated in the European Center for Medium-Range Weather Forecasts (ECMWF) since 1979 is used as anatmospheric tracerto study the modulations of the winter Arctic stratosphere by the quasi-biennial oscillation (QBO) and the solar cycle. It is found that both the QBO and solar forcing in low latitudes can perturb the late winter polar vortex, likely via planetary wave divergence, causing an early breakdown of the vortex in the form of Stratospheric Sudden Warming. As a result, TCO within the vortex in late winter can increase by ~60 DU during either a solar maximum or an easterly phase of the QBO, or both, relative to the least perturbed state when the solar cycle is minimum and the QBO is in the westerly phase. In addition, from the solar maximum to solar minimum during the QBO easterly phase, the change in TCO is found to be statistically insignificant. Therefore, the ‘reversal’ of the Holton–Tan effect, reported in some previous studies using lower stratospheric temperature, is not evident in the TCO behavior of both observation and assimilation.

“The top down solar effect on the jets is provided via the size and intensity of the atmospheric polar vortices, one at each pole. That size and intensity is set by the height of the tropopause at the poles. When the tropopause rises the polar vortex becomes deeper but less extensive at the surface (jets shift poleward). When the tropopause at the poles falls the polar vortex becomes shallower but more extensive at the surface (jets shift equatorward).”